TDM and WDM

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TDM and WDM
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Time Division Multiplexing

• Type of digital or (rarely) analog multiplexing
  in which two or more signals or bit streams are
  transferred apparently simultaneously as
  sub-channels in one communication channel
• The time domain is divided into several recurrent
  timeslots of fixed length, one for each
  sub-channel.
• A sample byte or data block of sub-channel 1 is
  transmitted during timeslot 1, sub-channel 2
  during timeslot 2, etc.

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Technology Trends TDM,DWDM
Optical
Electrical
Time Division Multiplexing (TDM)
Optical
Electrical
Dense Wavelength Division Multiplexing (DWDM)
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Transmission using TDM

• In circuit switched networks such as the public
  switched telephone network (PSTN) there exists
  the need to transmit multiple subscribers calls
  along the same transmission medium.
• TDM allows switches to create channels, also
  known as tributaries, within a transmission
  stream.
• A standard DS0 voice signal has a data bit rate
  of 64 kbit/s, determined using Nyquists sampling
  criterion.
• TDM takes frames of the voice signals and
  multiplexes them into a TDM frame which runs at a
  higher bandwidth.
• If the TDM frame consists of n voice frames, the
  bandwidth will be n64 kbit/s.
• Each voice sample timeslot in the TDM frame is
  called a channel

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Transmission using TDM

• In European systems, TDM frames contain 30
  digital voice channels.(2.048 Mbps )
• In American systems, TDM frames contain 24
  channels.(1.544 Mbps)
• Both standards also contain extra bits (or bit
  timeslots) for signalling and synchronization
  bits.
• Multiplexing more than 24 or 30 digital voice
  channels is called higher order multiplexing.
• Higher order multiplexing is accomplished by
  multiplexing the standard TDM frames.
• For example, a European 120 channel TDM frame is
  formed by multiplexing four standard 30 channel
  TDM frames.
• At each higher order multiplex, four TDM frames
  from the immediate lower order are combined,
  creating multiplexes with a bandwidth of n x 64
  kbit/s, where n 120, 480, 1920, etc.

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T1 Frame
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E1 Frame
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Digital transmission hierarchy
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North American Digital Hierarchy
European Digital Hierarchy
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SONET/SDH hierarchy.
Optical Carrier SONET/SDH Signal Bit Rate Capacity
OC-1 STS-1 51.84 Mbps 28 DS1s or 1 DS3
OC-3 STS-3/STM-1 155.52 Mbps 84 DS1s or 3 DS3s
OC-12 STS-12/STM-4 622.08 Mbps 336 DS1s or 12 DS3s
OC-48 STS-48/STM-16 2488.32 Mbps 1344 DS1s or 48 DS3s
OC-192 STS-192/STM-64 9953.28 Mbps 5379 DS1s or 192 DS3s
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SONET/SDH

• SDH also performs some switching functions
• SDH Crossconnect
• The SDH Crossconnect is the SDH version of a
  Time-Space-Time crosspoint switch.
• It connects any channel on any of its inputs to
  any channel on any of its outputs.
• SDH Add-Drop Multiplexer
• The SDH Add-Drop Multiplexer (ADM) can add or
  remove any multiplexed frame down to 1.544Mb.

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TDM

• Synchronous time division multiplexing
• Uses fixed time slots
• Asynchronous/Statistical time division
  multiplexing
• Logically distribute bandwidth

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Statistical TDM

• STDM allows bandwidth to be split over 1 line.
• Many college and corporate campuses use this type
  of TDM to logically distribute bandwidth.
• A more common use however is to only grant the
  bandwidth when it is needed.
• STDM does not reserve a time slot for each
  terminal, rather it assigns a slot when the
  terminal is requiring data to be sent or
  received.
• This is also called asynchronous time-division
  multiplexing(ATDM)

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Why WDM

• SDH network functions are connected using
  high-speed optic fibre.
• Optic fibre uses light pulses to transmit data
  and is therefore extremely fast.
• Modern optic fibre transmission makes use of
  Wavelength Division Multiplexing (WDM) where
  signals transmitted across the fibre are
  transmitted at different wavelengths, creating
  additional channels for transmission.
• This increases the speed and capacity of the
  link, which in turn reduces both unit and total
  costs

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Why WDM?

• Capacity upgrade of existing fiber networks
  (without adding fibers)
• Transparency Each optical channel can carry any
  transmission format (different asynchronous bit
  rates, analog or digital)
• Scalability Buy and install equipment for
  additional demand as needed
• Wavelength routing and switching Wavelength is
  used as another dimension to time and space

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Active Components Tunable Optical Filters,
Tunable Sources, Optical Amplifiers Passive
Components require no external control for their
operation, so have limited applicaiton
Typical WDM Link using a
variety of passive and active devices.
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WDM, CWDM and DWDM

• WDM technology uses multiple wavelengths to
  transmit information over a single fiber
• Coarse WDM (CWDM) has wider channel spacing (20
  nm) low cost
• Dense WDM (DWDM) has dense channel spacing (0.8
  nm) which allows simultaneous transmission of 16
  wavelengths high capacity

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WDM

• WDM SYSTEM

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WDM SYSTEMS

• Four kinds of WDM systems are available
• Metro WDM (lt200 km)
• Long-haul or regional WDM (200 km to 800 km)
• Extended long-haul WDM (800 km to 2000 km)
• Ultra-long-haul WDM (gt2000 km)

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Coarse Wavelength-Division Multiplexing

• The short-haul transport of data, voice, video,
  storage, and multimedia services
• CWDM systems use lasers that have a bit rate of
  up to 2.5 Gbps (OC-48/STM-16) and can multiplex
  up to 18 wavelengths. This provides a maximum of
  45 Gbps over a single fiber.
• Channel spacing of 20 nm or 2500 GHz as specified
  by the ITU standard G.694.2( 1270 nm to 1610 nm)

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Dense Wavelength-Division Multiplexing

• Metro or long-haul core where capacity demands
  are extremely high.
• Typical DWDM systems use lasers that have a bit
  rate of up to 10 Gbps (OC-192/STM-64) and can
  multiplex up to 240 wavelengths. This provides a
  maximum of 2.4 Tbps over a single fiber.
• Uses 100-GHz or 200-GHz frequency spacing.
• ITU grid DWDM products operate in the C-band
  between 1530 and 1565 nm or L-band between 1565
  and 1625 nm

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Optical frequency bands used with various WDM
systems

• O-band (original) A range from 1260 nm to 1360
  nm
• E-band (extended) A range from 1360 nm to 1460
  nm
• S-band (short wavelength) A range from 1460 nm
  to 1530 nm
• C-band (conventional) A range from 1530 nm to
  1565 nm
• L-band (long wavelength) A range from 1565 nm to
  1625 nm
• U-band (ultra-long wavelength) A range from 1625
  nm to 1675 nm

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Applications

• Consumer equipment Barcode scanner, printer,
  CD/DVD/, remote control devices
• Telecommunications Optical fiber communications,
  Optical Down converter to Microwave
• Medicine Correction of poor eyesight, laser
  surgery, surgical endoscopy, tattoo removal
• Industrial manufacturing The use of lasers for
  welding, drilling, cutting, and various methods
  of surface modification
• Construction Laser leveling, laser range
  finding, smart structures
• Aviation
• Military IR sensors, command and control,
  navigation, search and rescue, mine laying and
  detection
• Entertainment Laser shows, beam effects
• Information processing
• Metrology Time and frequency measurements, range
  finding
• Photonic computing clock distribution and
  communication between computers, circuit boards,
  optoelectronic integrated circuits